Inverters are used for converting direct current, for example produced by a photovoltaic generator (PV generator) in a photovoltaic installation (PV installation), into an alternating current that is suitable for supply to a power supply grid. In light of the increasing prevalence of regenerative power generating installations, particularly PV installations, the demands of power supply companies on the functionality of the inverter, particularly in respect of grid-supporting operation, are rising. The operators of power supply grids frequently have the demand, specified in what is known as the Grid Code, that regenerative power generating installations are not, as usual in the past, shut down in the event of a grid failure, for example in the event of voltage drops, but rather must be able to ride through the grid failure (fault ride through—FRT). This means that, on the one hand, at the end of the grid failure, power can be supplied to the power supply grid again as immediately as possible and, on the other hand, during the grid fault, the power supply grid can have its voltage supported by a supply of reactive current. By way of example, a grid fault occurs if the amplitude or the RMS value of a single-phase system grid voltage is below a minimum value. In the case of polyphase power supply grids, a similar definition can be provided on the basis of the amplitude mean values of the individual phases, for example. On account of the significantly reduced grid voltage in such a case, generation of the demanded reactive and/or active current requires only a small RMS output voltage from the inverters.
A rather short grid fault that is intended to be ridden through in order to support the grid (FRT) must be distinguished from a grid fault that results in the formation of an islanding situation for the PV installation. Such an islanding situation is characterized by complete or almost complete outage of the power supply grid. In such a situation, there is usually provision for the PV installation to be shut down, e.g. for reasons of protection against electric shock. It is alternatively also possible for the local loads connected to the inverter to be supplied with power, if the PV installation and particularly the inverters thereof are arranged for this.
A method of detecting and discriminating between a grid failure that needs to be ridden through and an islanding situation practiced up to now is based, by way of example, on the attempt to actively influence the current in the power supply grid, for example to alter the phase of the current. Depending on the reaction of the power supply grid to this disturbance attempt, it is possible to infer one or the other state of the grid.
The document US 2013/0155734 A1 discloses a method in which different measured electrical parameters of a power supply grid are used in order to determine whether there is possibly an islanding situation in the power supply grid. Suitable parameters that are mentioned are a voltage measured in the power supply grid, a current, a frequency or an impedance. If there is a potential islanding situation, a supply of active and/or reactive power to the power supply grid is increased in order then to take a fresh measurement of one or more of the mentioned parameters as a basis for checking the forecast of the first measurement.
As the prevalence of supplying PV installations in power supply grids increases, these methods, like other active methods for detecting islanding, reach their limits. The reason is that the inverters disturb each other with their methods for detecting islanding, and these methods increasingly lead to unreliable results or an unstable operating state.